The present invention relates to Marx generators and, more particularly, to an improved trigger circuit for triggering a Marx generator.
X-ray machines that generate X-rays from cold field emission of electrons from the cathode of an X-ray tube are commonly employed in pulsed shadowgraph radiographs. Pulsed or flash shadowgraph radiograph was developed in 1938 as a means for observing extremely rapid motion where the subject was obscured from observation with visible light or debris. To date, flash radiography remains the principal means of observing lensed implosions and ballistic impacts over microsecond and nanosecond time scales. The majority of these X-ray systems utilize the well known Marx generator which can be viewed as a distributed transmission-storage line, consisting of n-cascaded high-voltage capacitors each connected across an associated spark gap switch. To produce X-rays, the Marx generator is coupled to a field emission X-ray tube either directly or by coaxial cables and is triggered to initiate a spark discharge at the surface of the positive electrode of the first spark gap switch.
In one prior art triggering technique, a separate ultraviolet (UV) illuminator has been employed in combination with a separate charging capacitor. This capacitor is charged from the Marx high-voltage supply. It has been found that at low Marx charging voltages, the UV pulse from the illuminator does not reliably photoionize the first spark gap switch.
In our earlier publication, Boyer et al, Pulsed Hard X-Ray Source for Nondestructive Testing and Medical Imaging, Proc. SPIE, Vol. 3154, p. 16-26 (1997), we disclose a triggering arrangement wherein the first spark gap switch is modified by use of a three electrode xe2x80x9ctrigatronxe2x80x9d switch described below. The switch includes a first, host electrode having incorporated therein an insulator containing a central trigger pin, and a second, grounded electrode. A 25 kV pulse discharge between the trigger pin and the host electrode provides rapid closure of the gap between the first and second electrodes.
In addition to our own earlier patents, other patents of interest include U.S. Pat. No. 5,311,067 to Grothaus et al. which discloses a Marx-type generator for producing a high voltage burst of pulses having a high repetition ratio and including a trigatron switch, and U.S. Pat. No. 3,256,439, which discloses a high voltage and high current pulse generator combined with an X-ray tube and including ball electrodes defining a spark gap and a triggering electrode.
In accordance with the invention, an improved trigger circuit for the triggering arrangement of our earlier publication (Boyer et al) is provided. The trigger can be made quite small so that the volume of the overall trigger generator can be reduced to a few cubic inches. This permits connection of the generator through a short cable having little parasitic capacitance. The resultant reduction in the risetime of the high voltage pulse reduces the trigger jitter of the associated Marx generator.
In accordance with the invention, a triggering circuit is provided for a triggering system for a Marx generator column comprising a plurality of metal electrode pairs wherein the electrodes of each pair are spaced to form a spark gap therebetween and a capacitor is connected across each gap, the triggering system comprising a three electrode spark gap switch forming the first spark gap of the Marx generator column and the trigger circuit comprising: a trigger transformer having a primary winding and a secondary winding, the secondary winding being connected to the three electrode spark gap through a connecting cable; an oscillator power supply for producing an output voltage; a charging capacitor connected to the oscillator power supply so as to be charged thereby to said output voltage; and an electronic switch connected to a junction between the charging capacitor and said oscillator power supply for, when actuated by an input pulse, completing a connection between the capacitor and the primary winding of the trigger transformer so that a high voltage trigger pulse is produced at the secondary winding of the trigger transformer.
Advantageously, the trigger further comprises a pulse generator circuit connected to an electronic switch for, responsive to receiving a trigger pulse, generating said input pulse for actuating the electronic switch. Preferably, the pulse generator circuit includes a further electronic switch including an input terminal for receiving said trigger pulse and an output terminal, and a further transformer including a primary winding connected to the output terminal of the second electronic switch, and a secondary winding connected to a control input of the first electronic switch. Advantageously, a resistor is connected in a series circuit including said output terminal of the second electronic switch and the primary winding of the further transformer. A further resistor is preferably connected in parallel with the series circuit of the resistor and the primary winding.
Preferably, a common supply bus is connected to the second electronic switch and to the oscillator power supply. A radio frequency choke is advantageously connected between the supply bus and the oscillator power supply.
Preferably, the second electronic switch comprises a transistor having a base and an emitter-collector circuit, and a NAND gate is connected to the base of the transistor. Advantageously, the emitter-collector circuit of the transistor is connected between the common supply bus and the secondary winding of the second transformer. A stabilizing capacitor is preferably connected in a connection between the emitter of said transistor and ground. The first mentioned electronic switch preferably comprises a silicon controlled rectifier. A diode is advantageously connected in parallel with the charging capacitor.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.